High brightness dense wavelength multiplexing laser

ABSTRACT

The present disclosure describes systems and methods for beam wavelength stabilization and output beam combining in dense wavelength multiplexing (DWM) systems. Systems and methods are described for performing beam wavelength stabilization and output beam combining in DWM systems while achieving increased wall-plug efficiency and enhanced beam quality. Interferometric external resonator configurations can be used to greatly increase the brightness of DWM system output beams by stabilizing the wavelengths of the beams emitted by the emitters of the DWM laser source. The resonant cavities described by the present disclosure provide advantages over the prior art in the form of decreased cost, increased wall plug efficiency and increased output beam quality. Particular implementations of the disclosure achieve increased wall plug efficiency and increased output beam quality through a combination of innovative cavity designs and the utilization of reflection diffraction elements for beam combining.

FIELD OF THE INVENTION

The present disclosure relates generally to laser systems and moreparticularly to systems and methods for narrow-bandwidth laser beamstabilization and multiple laser beam combining.

BACKGROUND OF THE INVENTION

Dense wavelength multiplexing is a technique for producing a single,high-brightness, multi-spectral, combined output beam from a pluralityof individual, narrow spectral bandwidth input beams. DWM enablesmultiple relatively low-power single wavelength input beams to besuperimposed to produce a single, high-power, high-brightness outputbeam. DWM techniques enable output beam power to be scaled directly withthe sum of the power produced by the plurality of input beams and tofurther provide output beams with beam quality commensurable with thebeam quality of the individual input beams.

In DWM systems, a plurality of narrow spectral bandwidth, or singlewavelength, laser beams are emitted from a laser source that comprises aplurality of individual emitters. The multi-spectral output beam isformed by combining, or spatially and directionally overlapping, theplurality of individual beams with a beam overlapping element. Beamcombining can be achieved through selecting a single wavelength for eachof the beams and directing each of the beams at the beam overlappingelement with a particular angle of incidence. The wavelength and angleof incidence of each beam is selected such that all of the beams emergefrom the beam overlapping element at an overlap region with a commondirection of propagation. A set of allowed wavelength-angle pairs can bedefined as all combinations of wavelength and angle of incidence thatwill yield a beam that emerges from the beam overlapping element at thecommon direction of propagation.

In order to produce a single multi-spectral combined output beam fromthe plurality of laser beams emitted by the laser source, awavelength-angle pair from the set of allowed wavelength-angle pairsmust be selected for each emitter in the laser source. Angle ofincidence selection can be accomplished by fixing the relative positionof the laser source and beam overlapping element and placing aposition-to-angle transformation lens at a fixed position in the opticalpath between the laser source and the beam overlapping element. Theposition-to-angle transformation lens maps the spatial position of eachemitter in the laser source to a particular angle of incidence at thebeam overlapping element.

For each individual emitter, wavelength selection can be accomplished byproviding feedback to the emitter in the form of electromagneticradiation with the desired wavelength. Providing such electromagneticradiation to the emitter will excite a resonant mode of the emittercorresponding to the desired output. Thus, providing feedback to theemitter will stimulate the emission of additional electromagneticradiation with a wavelength that is equivalent to that of the feedback.The resonant feedback will narrow the spectral bandwidth of the laserbeam emitted by the emitter and center the wavelength spectrum of theemitted beam about the wavelength of the resonant feedback. This processof providing feedback to an emitter can be referred to as beamwavelength stabilization, or wavelength locking.

Locking the wavelength of each laser beam maps a single wavelength toeach position of an emitter in the laser source and creates a set offixed wavelength-position pairs for the laser source. Theposition-to-angle transformation lens maps the wavelength-position pairfor each emitter in the laser source to a particular wavelength-anglepair. Selecting appropriate wavelength-position pairs ensures that aspatially and directionally overlapped beam will be produced. However,if the wavelength locking is not robust and alternative resonant modesof the individual emitters are excited, the emitters will producealternative resonant mode components that will thereafter propagatethrough the system. The beam components produced by the alternativeresonant modes do not represent allowed wavelength-position pairs andwill therefore not be spatially and directionally overlapped by the beamoverlapping element. Furthermore, if such alternative resonant modes areallowed to propagate through an external resonator that providesfeedback to the laser source in order to stabilize the wavelength of thebeams emitted by the emitters in the laser source, these alternativeresonant modes will stimulate the emission of further parasitic,alternative mode components and thereby degrade output beam quality andinduce temporal fluctuations in output beam power.

SUMMARY OF THE INVENTION

The present disclosure describes systems and methods for beam wavelengthstabilization and output beam combining in DWM systems. The presentdisclosure more specifically describes systems and methods forperforming beam wavelength stabilization and output beam combining inDWM systems while achieving increased output beam brightness, increasedwall plug efficiency, and enhanced beam quality relative to the priorart. Wall plug efficiency is a measure of the efficiency with which thesystem converts electrical power into optical power and can be definedas the ratio of the radiant flux, i.e. the total optical output powerproduced by the system, to the input electrical power consumed by thesystem.

Some prior art systems and methods for beam stabilization and outputcombining utilize transmissive optical elements, and specificallytransmissive diffraction gratings, for beam combining purposes. The useof transmissive diffraction elements for beam combining purposes suffersfrom a number of limitations. Beam stabilization and output combiningsystems and methods that utilize transmissive diffraction elementspresent certain disadvantages. For one while the theoretical diffractionefficiency of transmission diffraction elements is very close to onehundred percent, real transmissive diffraction elements cannot achievesuch diffraction efficiency in practice. Real transmissive diffractionelements can consistently achieve diffraction efficiencies notsubstantially greater than about ninety-five percent. Therefore, it isnecessary to design external cavities capable of capturing diffractionorders that would not otherwise be used in order to maximize wall-plugefficiency. By contrast, reflection diffraction elements are capable ofachieving substantially higher diffraction efficiencies. In practice,reflection diffraction elements can achieve diffraction efficiencies ofup to roughly 99.8%. Resonant cavities utilizing reflection diffractionelements can thereby attain an appreciable increase in wall-plugefficiency relative to external resonators that utilize transmissivediffraction elements.

Second as laser beams propagate through transmissive diffractionelements, the transmissive elements absorb a small amount ofelectromagnetic radiation. Additionally, antireflective coatingscovering the front and rear faces of the transmissive diffractionelements absorb a non-insignificant amount of electromagnetic radiationfrom incident laser beams. The absorption of the electromagneticradiation generates heat within the regions of the transmissive elementsthrough which the electromagnetic radiation passes. The heat generatedby absorption of radiation will propagate towards the periphery of thetransmissive elements and a heat-flow gradient will be created withinthe transmissive element. Such a heat-flow gradient can degrade beamquality and decrease the wall-plug efficiency of the system. At lowoutput power, degradation to beam quality may be insignificant andwall-plug efficiency losses may be relatively minor. However, as beamoutput power is increased, the generation of heat within thetransmissive elements causes more significant beam distortions andefficiency losses. At very high power, e.g. on the order of ˜several kW,beam distortions induced by heat generation become significant.Reflection diffraction elements exhibit extremely low absorption ofelectromagnetic radiation from incident laser beams and thereby enablepower scalability without loss of beam quality or efficiency.

Third, as the output power produced by the system increases, it maybecome necessary to cool diffraction elements in the system the effectsof heat generation become more significant. The geometries of reflectivediffraction elements allow for superior cooling techniques as comparedto those allowed by transmissive diffraction elements. Geometricalconstraints dictate that any cooling system employed alongside atransmissive diffraction element must be situated at an outer boundary,i.e. at the periphery, of the transmissive element. Specifically, thepossibility of placing a cooling system on either face of a transmissiveelement is precluded because radiation enters a transmissive element atone face and exits the element at an opposite face. Cooling systemslocated at the periphery of transmissive gratings increase the magnitudeof thermal gradients by actively drawing heat away from the center ofthe element and towards the periphery. Peripheral cooling systemsthereby induce thermo optical phase distortions in the wave front of theincident laser beams. These induced phase distortions result indecreased diffraction efficiency, changes in beam properties, localchanges in the grating constant, and most importantly, dynamic thermallensing. These effects further decrease the wall-plug efficiency of thesystem and further deteriorate output beam quality.

By contrast, reflective diffraction elements allow thermal managementtechniques capable of achieving substantially one-dimensional (in adirection perpendicular to the plane in which the surface of thediffraction element lies) heat-flow in the diffractive element. Onedimensional heat-flow suppresses thermal lensing effects and eliminateslocal differences in the grating constant that result from thermalgradients within the grating. One dimensional heat-flow can be achievedthrough selecting a diffractive element with an appropriatethickness-to-diameter ratio and the use of a cooling element located ona face of the diffractive element opposite the face at which incidentradiation is reflected. The cooling element (i.e. heat sink) ispreferably placed in “form-fitting” contact with the grating and mayoffer an added benefit by providing additional mechanical stability forthe device thereby preventing the grating from bending under thermalload. For example, a relatively thin reflective diffraction grating canbe bonded to a heat sink with comparatively high heat conductivity. Insuch a configuration, the thin reflective diffraction grating will actas the main barrier for heat transport and the position of the heat sinkrelative to grating will facilitate a uniform distribution of thermalenergy within the grating itself

In addition to enabling advantageous geometric positioning of coolingelements, reflective diffraction elements need not be formed fromoptically transparent materials. Therefore, reflective diffractionelements allow for improved cooling properties through the use of avariety of materials with high heat conductivity and or low coefficientsof thermal expansion, e.g. diamond, sapphire, glass-ceramics (e.g.Zerodur), and Zinc Sulfide. Such materials are typically not opticallytransparent, or not of optical grade, and therefore cannot be used intransmissive diffraction elements without loss in power or beam quality.

One embodiment of the present invention provides a system forstabilizing the wavelength of beams emitted by a plurality of beamemitters, the system comprising the plurality of beam emitters each beamemitter emitting a beam, a first reflection diffraction element, and afeedback branch comprising a spatial filtering system, wherein the firstreflection diffraction element directs a portion of the beamsoriginating at the array into the feedback branch as feedback branchinput, and wherein the feedback branch directs a portion of the feedbackbranch input back into the plurality of beam emitters.

An alternative embodiment of the present invention provides a method forstabilizing the wavelength of beams emitted by a plurality of beamemitters, the method comprising directing the emitted beams towards afirst reflection diffraction element, directing a portion of the emittedbeams from the first reflection diffraction element into a feedbackbranch as a feedback branch input, and directing a portion of thefeedback branch input through the feedback branch and back into theplurality of beam emitters, wherein directing a portion of the feedbackbranch input through the feedback branch comprises sequentiallydirecting a portion of the feedback branch input through a spatialfiltering system.

An additional embodiment of the present invention provides a densewavelength multiplexing system comprising an array of beam emitters eachemitting a single wavelength beam, a first diffraction elementreflecting the plurality of single wavelength beams from the array ofbeam emitters, and a second diffraction element diffracting thereflection of the plurality of single wavelength beams so as to combinethe beams into a single multi-wavelength combined beam.

Another additional embodiment of the present invention provides a densewavelength multiplexing system comprising a plurality of beam emitterseach emitting a single wavelength beam and a first diffraction elementdiffracting a portion of the beams from the plurality of beam emittersso as to combine the beams into a single multi-wavelength combinedoutput beam.

A further embodiment of the present invention provides a densewavelength multiplexing and beam wavelength stabilization systemcomprising a plurality of beam emitters each emitting a beam, at leastone optical element, a first reflection diffraction grating, a secondreflection diffraction grating, and a feedback branch having a first armand a second arm, wherein the first reflection diffraction gratingdirects a reflection of the beams towards the second reflectiondiffraction grating and directs a diffraction of the beams into thefirst arm of the feedback branch as a first arm input, wherein thesecond reflection diffraction grating receives the reflection of thebeams and diffracts the reflection as an output, wherein the first armof the feedback branch includes a first highly reflective mirrorpositioned to reflect the first arm input back toward the firstreflection diffraction grating as a first arm output such that the firstreflection diffraction grating diffracts a first portion of the firstarm output back to the array of beam emitters as a first feedbackportion and reflects a second portion of the first arm output into thesecond arm as a second arm input, and wherein the second arm includes asecond highly reflective mirror positioned to reflect the second arminput back to the first reflection diffraction grating as a second armoutput such that the first reflection diffraction grating reflects afirst portion of the second arm output into the first arm andreflectively diffracts a second portion of the second output arm outputtowards the second reflection diffraction grating.

Another further embodiment of the present invention provides a densewavelength multiplexing and beam wavelength stabilization systemcomprising a plurality of beam emitters each emitting a beam, at leastone optical element, a reflection diffraction grating, and a feedbackbranch having a first arm and a second arm, wherein the reflectiondiffraction grating directs a reflection of the beams into the first armof the feedback branch as a first arm input and directs a diffraction ofthe beams as a system output, wherein the first arm of the feedbackbranch includes a first highly reflective mirror positioned to reflectthe first arm input back toward the reflection diffraction grating as afirst arm output such that the reflection diffraction grating reflects afirst portion of the first arm output back to the array of beam emittersas a first feedback portion and diffracts a second portion of the firstarm output into the second arm as a second arm input, and wherein thesecond arm includes a second highly reflective mirror positioned toreflect the second arm input back to the reflection diffraction gratingas a second arm output such that the reflection diffraction gratingreflects a first portion of the second arm output as an additionalsystem output and diffracts a second portion of the second output armoutput into the first arm of the feedback branch as an additional firstarm input.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an apparatus for producing a single, multi-wavelengthoutput laser beam comprising a plurality of spatially and directionallyoverlapped single wavelength beams.

FIG. 2 illustrates an array of beam emitters and an emitter arraytransformation arm of an interferometric external resonator.

FIG. 3 illustrates an interferometric external resonator and beamcombining apparatus that utilizes a reflection diffraction element toprovide resonant feedback for beam wavelength stabilization and tocombine multiple single-wavelength beams into a combined,multi-wavelength output beam.

FIG. 4 illustrates an alternative interferometric external resonator andbeam combining apparatus that utilizes reflection diffraction elementsto provide resonant feedback for beam wavelength stabilization and tocombine multiple single-wavelength input beams into a combined,multi-wavelength output beam.

FIG. 5 illustrates an interferometric external resonator and beamcombining apparatus that utilizes a high diffraction efficiencyreflection diffraction element to provide resonant feedback for beamstabilization and to combine multiple single wavelength input beams intoa combined, multi-spectral output beam.

FIG. 6 illustrates an additional interferometric external resonator andbeam combining apparatus that utilizes a high diffraction efficiencyreflection diffraction element with a backside thermal management systemto provide resonant feedback for beam stabilization and to combinemultiple single wavelength input beams into a combined, multi-spectraloutput beam.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates an apparatus for producing a single, multi-wavelengthoutput laser beam comprising a plurality of spatially and directionallyoverlapped single wavelength beams. The apparatus depicted in FIG. 1utilizes a reflection diffraction element to couple a plurality ofsingle wavelength beams, each of which is emitted by an individualemitter in a beam emitter array. The apparatus depicted in FIG. 1includes a laser source 101 and an output combining element 113. Thelaser source 101 includes a plurality of laser emitters. In variousimplementations, the laser source 101 may be a one dimensional array ofdiode laser emitters, a two dimensional array of diode laser emitters,or a one of a variety of additional configurations of laser emitters.The apparatus depicted in FIG. 1 further includes four optical patharms: an emitter array transformation arm 150, a first feedback brancharm 160, a second feedback branch arm 170, and an output arm 180. Theemitter array transformation arm 150, the first feedback branch arm 160,and the second feedback branch arm 170 together define an externalresonator cavity. The external resonator cavity provides feedback to thelaser source 101 in order to stabilize the wavelength of the beamsemitted by the laser source 101. Feedback efficiency of the externalresonator cavity is defined as the ratio of optical power coupled into awaveguide of an emitter in the laser source to the total optical powertransmitted through the external resonator cavity and towards the lasersource 101. It is preferably to achieve high feedback efficiency inorder to stabilize the wavelengths of the beams emitted by the pluralityof emitters in the laser source. Specifically, the feedback efficiencyshould be in excess of thirty percent and preferably in excess ofseventy percent.

The apparatus depicted in FIG. 1 further includes a position-to-angletransformation element 102, a first reflection diffraction element 103,a first optic 104 and a second optic 106 of a first feedback branch armtelescope, a filtering element 105, a first highly reflective mirror 107of the first feedback branch arm, a first optic 108 and a second optic109 of a second feedback branch arm telescope, a second highlyreflective mirror 110 of the second feedback branch arm, and a firstoptic 111 and a second optic 112 of an output arm telescope. In theembodiment depicted in FIG. 1, the first optic 104 and the second optic106 of the first feedback branch arm and the first optic 108 and thesecond optic 109 of the second feedback branch arm are Fourier lenses.However, in alternative implementations, Fresnel lenses and variousmirror configurations may be utilized.

In the apparatus depicted in FIG. 1, the first reflection diffractionelement 103 functions as a resonant feedback component coupling element,while the second reflection diffraction grating functions as a beamcombining element for the output beam. Other implementations of theinvention may utilize a variety of different external resonator and beamcombining system configurations. Alternative configurations may include,but are not limited to, configurations having optical paths with threebranches (such as the resonator and beam coupler depicted in FIG. 3) aswell as configurations having optical paths with five branches or withany other number of branches. Alternative configurations may alsoinclude, but are not limited to, configurations utilizing a reflectiondiffraction grating with a high diffraction efficiency in the firstorder for both resonant feedback component coupling and output beamcombining (such as the resonator and beam combining apparatus depictedin FIG. 5).

FIG. 2 illustrates an array of beam emitters and an emitter arraytransformation arm of an interferometric external resonator. The emitterarray 201 depicted in FIG. 2 is an array of diode laser emitters. Thearray of diode laser emitters may comprise a stack of diode bars, whereeach bar has a plurality of emitters. The array can also be eitherone-dimensional or two-dimensional. Typically, each emitter in the diodebar has an asymmetric beam profile for which two axes perpendicular tothe direction in which the beam propagates can be defined. The two axescan be identified as a fast axis and a slow axis and are alsoperpendicular to each other. The laser beam will diverge more rapidlyalong the fast axis and more slowly along the slow axis. In alternativeimplementations, the emitter array 201 may comprise a plurality of gaslasers, chemical lasers, excimer lasers, solid-state lasers, photoniccrystal lasers, dye lasers, or any other type of laser source.

A plurality of cylindrical fast-axis collimation optics 202 collimateeach of the beams emitted by the diode laser emitters in the array ofdiode laser emitters 201 along the fast axis. A plurality of beamrotators 203 rotates each of the beams emitted by the diode laseremitters in the array of diode laser emitters 201. Each beam rotatorrotates its respective beam by ninety degrees in a directionperpendicular to the direction of propagation. Alternatively the devicecan be set up without using beam rotators. In this case it might beadvantageous to use slow axis collimation lens arrays to increase theoptical filling factor in the slow axis.

A cylindrical slow-axis collimation optic 204 collimates each of therotated beams emerging from the plurality of beam rotators 203 along thebeams slow axis. A position-to-angle transformation element 206transforms the spatial distribution of the beams emitted by the diodelaser emitters in the array of diode laser emitters 201 to an angulardistribution. The position-to-angle transformation element 206 may be alens, a prismatic object, or any other element than can impart an angleof transmission upon a beam that is determined by the position at whichthe beam is incident upon the element. The position-to-angletransformation element 206 imparts each beam emitted by the array ofdiode laser emitters 201 and emerging from the first cylindrical slowaxis telescope optic 205 with an angle of incidence with respect to areflection diffraction grating 208. In some implementations, the angleof incidence imparted by the position-to-angle transformation element206 is selected such that all of the beams will be spatially anddirectionally overlapped after being diffracted from the reflectiondiffraction grating 208 as diffracted emitter array transformation armoutput 209. Alternatively, the angle of incidence imparted by theposition-to-angle transformation element 206 is selected such thatreflected emitter array transformation arm output 210 will be spatiallyand directionally overlapped after being diffracted by a subsequentdiffraction element. In some implementations, the array of diode beamemitters 201 is positioned such that each emitter in the array islocated at a particular angle with respect to the reflection diffractiongrating 208. In such implementations, each emitter emits a beam at thedesired angle with respect to the reflection diffraction grating 208 andtherefore a position-to-angle transformation element need not beincluded in the system.

When the emitter array transformation arm depicted in FIG. 2 isconfigured within a resonator cavity, a resonant feedback component 211traverses the emitter array transformation arm in the reverse direction.Depending upon the configuration of the other components of theresonator cavity, the resonant feedback component 211 incident upon thereflection diffraction grating 208 may either be a multi-spectral beamconsisting of a plurality of individual, single wavelength beams thatare spatially and directionally overlapped, or the resonant feedbackbeam 211 may consist of a plurality of individual, single-wavelengthbeams having an angular spectrum. In the former case, the reflectiondiffraction grating 208 will diffract the resonant feedback component211 and thereby transform the wavelength spectrum of the incident beaminto an angular spectrum. In the latter case, the reflection diffractiongrating 208 will reflect the resonant feedback component 211 andpreserve the preexisting angular spectrum. In either case, the resonantfeedback component 211 will comprise a plurality of single wavelengthbeams and will have an angular spectrum upon reaching theposition-to-angle transformation element 206. In the direction in whichthe resonant feedback component propagates, the position-to-angletransformation element 206 will act as an angle-to-positiontransformation element and impart a positional spectrum upon theresonant feedback component. The positional spectrum corresponds to thepositional arrangement of each of the emitters in the array of diodelaser emitters 201. In the emitter array transformation arm depicted inFIG. 2, the feedback efficiency is the ratio of the optical powercoupled into a waveguide of an emitter in the array of diode laseremitters 201 to the total optical power carried by the resonant feedbackcomponent 211.

FIG. 3 illustrates an interferometric external resonator and beamcombining apparatus that utilizes a reflection diffraction element toprovide resonant feedback for beam wavelength stabilization. FIG. 3illustrates an interferometric external resonator having two opticalpath arms: an emitter beam transformation arm 330 and a feedback arm335. The apparatus also has a third optical path arm (output arm 340)which is not part of the external resonator. A laser source 301 ispositioned at a first terminus of the emitter beam transformation arm330. The laser source 301 includes a plurality of emitters located atfixed positions with respect to one another. The position of each one ofthe plurality of emitters defines a point in an emitter spatial positiondistribution. The laser source 301 emits a plurality of singlewavelength, i.e. narrow spectral bandwidth, input beams that togetherconstitute an external resonator input 302.

Upon exiting the laser source, the external resonator input 302 has aposition spectrum that corresponds to the emitter spatial positiondistribution. The position spectrum maps each constituent beam of theexternal resonator input 302 to a particular emitter of the laser source301. The external resonator input 302 propagates from the laser sourcetowards a first reflection diffraction element 304 through aposition-to-angle transformation element 303. The position-to-angletransformation element 303 imparts an angle of incidence with respect tothe first reflection diffraction element 304 upon each constituent beamwherein the angles of incidence correspond to the spatial positions ofthe constituent beams. The position-to-angle transformation element 303thereby transforms the position spectrum of the external resonator input302 into an angular spectrum. Thus, upon emerging from theposition-to-angle transformation element 303, the external resonatorinput 302 possesses an angular spectrum. In the embodiment depicted inFIG. 3, the position-to-angle transformation element 303 is a Fourierlens. However, in alternative embodiments a variety of alternativetransformation optics, including Fresnel lenses, mirror arrangements,prismatic objects, and diffractive gratings may be utilized as theposition-to-angle transformation element 303.

After propagating through the position-to-angle transformation element303, the external resonator input 302 is split into separate componentsupon striking the first reflection diffraction element 304. In theembodiment depicted in FIG. 3, the first reflection diffraction element304 is a low diffraction efficiency diffraction grating. Therefore, themajority of the power of an incident beam emerges from the interactionwith the first reflection diffraction element as a reflection, while asubstantially smaller component emerges as a diffraction. In theembodiment depicted in FIG. 3, it is preferably that substantially allof the power of an incident beam emerge from the interaction with thediffraction grating as either a zero order diffraction (a reflection) ora first order diffraction. Furthermore, it is preferable that theoptical power carried by the reflection is at least four times theoptical power carried by the diffraction. In other words, in theembodiment depicted in FIG. 3, it is desirable to select a diffractiongrating that exhibits a first order diffraction efficiency of less thantwenty percent and that simultaneously exhibits a zero order diffractionefficiency of greater than eighty percent.

A first component emerging from the interaction between the externalresonator arm input 302 and the first reflection diffraction element 304is a reflection that constitutes an output arm input 305. The output arminput 305 consists of a plurality of single wavelength constituent beamsthat emerge from the interaction at various angles relative to oneanother. In other words, the output arm input 305 possesses an angularspectrum that is a reflection of the angular spectrum imparted upon theexternal resonator input 302 by the position-to-angle transformationelement 303. After emerging from the first reflection diffractionelement 304, the output arm input 305 strikes a second reflectiondiffraction element 306.

The second reflection diffraction element 306 is a high efficiency beamcombining reflection diffraction grating. The second reflectiondiffraction element reflectively diffracts the output arm input 305thereby converting the output arm input into a multi-spectral combinedoutput beam 307. The second reflection diffraction element exhibitsextremely high first order diffraction efficiency. In practice,reflection diffraction gratings can achieve diffraction efficiencies ofup to 99.8%. The use of a reflective diffraction element that exhibitsvery high diffraction efficiency results in very little loss in outputpower attributable to the output combining element and thereby enhancesthe wall-plug efficiency of the overall system.

The multi-spectral combined output beam 307 is composed of a pluralityof spatially and directionally overlapped single wavelength beams, whichare portions of the beams emitted from the plurality of emitters of thelaser source 301. In order to ensure that all constituent beams of themulti-wavelength combined output beam 307 share a common direction ofpropagation, the relative positions of the laser source 301, theposition-to-angle transformation element 303, the first reflectiondiffraction grating 304, the first and second output arm lenses 306 and307, and the second reflection diffraction element 306 must be fixed atprecise positions with respect to one another. Specifically, the opticalcomponents must be positioned such that the components of the pluralityof single wavelength beams emitted from the laser source 301 that reachthe second reflection diffraction element 306 emerge from an overlapregion of the second reflection diffraction element 306 with a commondirection of propagation.

A second component emerging from the interaction between the externalresonator input 302 and the first reflection diffraction element 304 isa diffraction that constitutes a feedback arm input 308. The feedbackarm input 308 includes a preferred resonant mode component 308A and analternative resonant mode component 308B. The preferred resonant modecomponent 308A is a combined beam composed of a plurality of spatiallyand directionally overlapped single wavelength constituent beams. Theconstituent beams of the preferred resonant mode component 308A arereflective diffractions of the constituent beams of the externalresonator input 302. The constituent beams of the preferred resonantmode component 308A are parallel, i.e. they emerge from the firstreflection diffraction element 304 with a common direction ofpropagation. Each constituent beam of the preferred resonant modecomponent 308A is composed of electromagnetic radiation corresponding toa preferred resonant mode of an emitter of the laser source 301. Thealternative resonant mode component 308B is composed of a plurality ofsingle wavelength constituent beams that emerge from the firstreflection diffraction element 304 at various angles with respect to thecommon direction of propagation of the preferred mode component 308A.Thus, the constituent beams of the alternative resonant mode component308B are not spatially and directionally overlapped with respect to eachother or with respect to the constituent beams of the preferred modecomponent 308A. In other words, the alternative resonant mode component308B has a broader angular spectrum than the preferred mode component308A. Consequently, alternative resonant mode component 308B willcontribute to a worsening of beam quality. Each constituent beam of thealternative resonant mode component 308B is composed of electromagneticradiation corresponding to an alternative, non-preferred mode of anemitter of the laser source 301. If the alternative resonant modecomponent 308B is allowed to propagate through the feedback arm andreturn to the laser source 301, the alternative resonant mode componentmay stimulate the emission of undesired spectral components from thelaser source 301. A portion of such undesired spectral components willbe transmitted out of the system and cause a deterioration in the beamquality of the multi-wavelength combined output beam 307.

After emerging from the first reflection diffraction grating 304, thefeedback arm input 308 travels through a first lens 309 of a feedbackarm telescope. The first lens 309 of the feedback arm telescope directsthe preferred mode component 308A through a spatial filtering element310 but directs the alternative resonant mode component 308B at thespatial filtering element 310 in a manner that causes the spatialfiltering element to block the alternative resonant mode component 308B.In this manner, the alternative resonant mode component 308B is filteredout of the external resonator and prevented from being returned to thelaser source 301 and thereby prevented from exciting alternativeresonant modes of the emitters of the laser source 301. Theelectromagnetic radiation composing the alternative resonant modecomponent 308B may be absorbed by the material composing the spatialfiltering element 310, or may be reflected from or transmitted throughthe spatial filtering element 310 in a manner such that it is divertedfrom the optical path defining the external resonator. In variousimplementations, the spatial filtering element may be a waveguidestructure, a set of mirrors that have a gradient layer, or an aperture,e.g., a diaphragm.

After passing through the spatial filtering element 310, the preferredmode component 308A propagates through a second lens 311 of the feedbackarm telescope and towards a feedback arm highly reflective mirror 312.The preferred mode component 308A is reflected from the feedback armhighly reflective mirror 312 as feedback arm output 313. Feedback armoutput 313 is a combined beam composed of a plurality of singlewavelength constituent beams, wherein each constituent beam is composedof electromagnetic radiation corresponding to a preferred resonant modeof an emitter of the laser source 301. After reflecting from thefeedback arm highly reflective mirror 312, the constituent beams of thefeedback arm output 313 travel in a reverse direction through the secondlens 311 of the feedback arm telescope, through the spatial filteringelement 310, and through the first lens 309 of the feedback armtelescope. Thereafter, the constituent beams of the feedback arm outputstrike the first reflection diffraction element 304. Upon striking thefirst reflection diffraction element 304, the feedback arm output 313 issplit into multiple separate components.

A first component emerging from the interaction between the feedback armoutput 313 and the first reflection diffraction element 304 is areflection that constitutes additional feedback arm input 308. Theadditional feedback arm input 308 is a combined beam composed of aplurality of parallel, e.g., directionally overlapped, and spatiallyoverlapped single wave length beams. A second component emerging fromthe interaction between the feedback arm output 313 and the firstreflection diffraction element 304 is a negative first order diffractionthat constitutes additional output arm input 305. A third componentemerging from the interaction between the feedback arm output 313 andthe first reflection diffraction element 304 is a first orderdiffraction that constitutes laser source resonant feedback 314. In theembodiment depicted in FIG. 3, the feedback efficiency is the ratio ofthe optical power coupled into waveguides of laser source 301 to thetotal optical power carried by the laser source resonant feedback 314.In order to promote wavelength stabilization of the emitters of thelaser source 301, the feedback efficiency should be thirty percent orgreater, and preferably greater than seventy percent. The diffraction ofthe feedback arm output 313 at the first reflective diffraction grating304 imparts an angular spectrum onto the laser source resonant feedback314. Specifically, the first reflective diffraction grating imparts anangle of diffraction upon each of the constituent beams of the lasersource resonant feedback 314. The laser source resonant feedback 314propagates through the position-to-angle transformation element 303towards the laser source 301 in a direction of propagation that isopposite to the direction of the external resonator input 302. Theposition-to-angle transformation element 303 transforms the angularspectrum of the laser source resonant feedback 314 into a positionspectrum that corresponds to the spatial distribution of the pluralityof emitters in the laser source 301. Thus, the position-to-angletransformation element 303 directs each constituent beam of the lasersource resonant feedback 314 into a single emitter of the laser source301 thereby stimulating emission of electromagnetic radiationcorresponding to the preferred resonant mode of each emitter of thelaser source 301.

FIG. 4 illustrates an alternative interferometric external resonator andbeam combining apparatus that utilizes reflection diffraction elementsto provide resonant feedback for beam wavelength stabilization and tocombine multiple single-wavelength input beams into a combined,multi-wavelength output beam. FIG. 4 illustrates an interferometricexternal resonator having three optical path arms: an emitter beamtransformation arm 430, a first feedback arm 435, and a second feedbackarm 440. The apparatus depicted in FIG. 4 also includes a fourth opticalpath arm (output arm 445) which is not part of the interferometricexternal resonator. A laser source 401 is positioned at a first terminusof the emitter beam transformation arm 430. The laser source 401includes a plurality of emitters located at fixed positions with respectto one another. The position of each one of the plurality of emittersdefines a point in the emitter spatial position distribution. The lasersource 401 emits a plurality of single wavelength, i.e. narrow spectralbandwidth, input beams that together constitute an external resonatorinput 402.

Upon exiting the laser source, the external resonator input 402 has aposition spectrum that corresponds to the emitter spatial positiondistribution. The external resonator input 402 propagates from the lasersource 401 towards a first reflection diffraction element 404 through aposition-to-angle transformation element 403. The position-to-angletransformation element 403 imparts an angle of incidence with respect tothe first reflection diffraction element 404 upon each constituent beamof the external resonator input 402. The imparted angles of incidencecorrespond to the spatial position of the constituent beams and thus tothe spatial position of the emitters in the laser source 401. Theposition-to-angle transformation element 403 thereby transforms theposition spectrum of the external resonator input 402 into an angularspectrum. Therefore, upon emerging from the position-to-angletransformation element 403, the external resonator input 402 possessesan angular spectrum. In the embodiment depicted in FIG. 4, theposition-to-angle transformation element 403 is a Fourier lens. However,in alternative embodiments a variety of alternative transformationoptics, including Fresnel lenses, mirror arrangements, prismaticobjects, and diffractive gratings may be utilized as theposition-to-angle transformation element 403.

After propagating through the position-to-angle transformation element403, the external resonator input 402 interacts with the firstreflection diffraction element 404. In the embodiment depicted in FIG.4, the first reflection diffraction element 404 is a low diffractionefficiency diffraction grating. Therefore, the majority of the power ofan incident beam emerges from the interaction with the first reflectiondiffraction element as a reflection, while a substantially smallercomponent emerges as a diffraction. In the embodiment depicted in FIG.4, it is preferably that substantially all of the power of an incidentbeam emerge from the interaction with the diffraction grating as eithera zero order diffraction (in which the beam reflected) or a first orderdiffraction. Furthermore, it is preferable that the optical powercarried by the reflection is at least four times the optical powercarried by the diffraction. In other words, in the embodiment depictedin FIG. 4, it is desirable to select a diffraction grating that exhibitsa first order diffraction efficiency of less than twenty percent andthat simultaneously exhibits a zero order diffraction efficiency ofgreater than eighty percent.

A first component emerging from the interaction between the externalresonator arm input 402 and the first reflection diffraction element 404is a reflection that constitutes an output arm input 405. The output arminput 405 is made up of a plurality of constituent beams and possessesan angular spectrum that is a reflection of the angular spectrumimparted upon the external resonator input 402 by the position-to-angletransformation element 403. After emerging from the first reflectiondiffraction element 404, the output arm input 405 travels through afirst output arm telescope lens 406 and a second output arm telescopelens 407 before striking a second reflection diffraction element 408.

The second reflection diffraction element 408 is a high efficiencyreflection diffraction element that acts as a beam combining element forthe output beam. Reflection diffraction elements can attain diffractionefficiencies considerably greater than those that transmissiondiffraction elements can attain. Therefore, the use of reflectiondiffraction elements provides increased wall-plug efficiency by reducingpower loss attributable to output beam combining The output arm input405 is converted into a multi-spectral combined output beam 409 viainteraction with the second reflection diffraction element 408. Themulti-spectral combined output beam 409 consists of a plurality ofspatially overlapped constituent beams that are diffractions of theconstituent beams of the output arm input 405. Furthermore, theconstituent beams of the multi-spectral combined output beam aredirectionally overlapped, i.e., they share a common direction ofpropagation. In order to ensure that the constituent beams of thecombined output beam are spatially overlapped and parallel, the relativepositions of the laser source 401, the position-to-angle transformationelement 403, the first reflection diffraction grating 404, the first andsecond output arm lenses 406 and 407, and the second reflectiondiffraction element 408 are fixed at precise positions with respect toone another.

A second component emerging from the interaction between the externalresonator arm input 402 and the first reflection diffraction element 404is a diffraction that constitutes a first feedback arm input 410. Thefirst feedback arm input 410 includes a preferred resonant modecomponent 410A and an alternative resonant mode component 410B. Thepreferred resonant mode component 410A is a combined beam composed of aplurality of spatially and directionally overlapped single wavelengthconstituent beams that emerge from the first reflection diffractionelement 404 with a common direction of propagation. The constituentbeams of the preferred resonant mode component 410A are reflectivediffractions of the constituent beams of the external resonator input402. Each constituent beam of the preferred resonant mode component 410Ais composed of electromagnetic radiation corresponding to a preferredresonant mode of an emitter of the laser source 401. The alternativeresonant mode component 410B is composed of a plurality of singlewavelength constituent beams that emerge from the first reflectiondiffraction element 404 at various angles with respect to the commondirection of propagation of the preferred mode component 410A. Thus, theconstituent beams of the alternative resonant mode component 410B arenot spatially and directionally overlapped with respect to each other orwith respect to the constituent beams of the preferred mode component410A. In other words, the alternative resonant mode component 410B has awider angular spectrum. Each constituent beam of the alternativeresonant mode component 410B is composed of electromagnetic radiationcorresponding to an alternative, non-preferred mode of an emitter of thelaser source 401. In other words, the alternative resonant modecomponents 410B is made up of unwanted spectral components of the firstfeedback arm input 410.

After emerging from the first reflection diffraction grating 404, thefirst feedback arm input 410 travels through a first lens 411 of a firstfeedback arm telescope. The first lens 411 of the first feedback armtelescope directs the preferred mode component 410A through a spatialfiltering element 412 but directs the alternative resonant modecomponent 410B into the spatial filtering element 412 such that it isdiverted from the optical path defining the external resonator. In thismanner, the alternative resonant mode component 410B is filtered out ofthe external resonator and prevented from being returned to the lasersource and thereby prevented from exciting alternative resonant modes ofthe emitters of the laser source 401. The electromagnetic radiationcomposing the alternative resonant mode component 410B may be absorbedby the material composing the spatial filtering element 412, or may bereflected from or transmitted through the spatial filtering element 412in a manner such that it is diverted from the optical path defining theexternal resonator.

After passing through the spatial filtering element, the preferred modecomponent 410A propagates through a second lens 413 of the firstfeedback arm telescope and towards a first feedback arm highlyreflective mirror 414. The preferred mode component 410A is reflectedfrom the first feedback arm highly reflective mirror 414 as firstfeedback arm output 415. First feedback arm output 415 is a combinedbeam composed of a plurality of single wavelength constituent beams.Each constituent beam of the first feedback arm output 415 is composedof electromagnetic radiation corresponding to a preferred resonant modeof an emitter of the laser source 401. After reflecting from the firstfeedback arm highly reflective mirror 414, the constituent beams of thefirst feedback arm output 415 travel in a reverse direction through thesecond lens 413 of the first feedback arm telescope, through the spatialfiltering element 412, and through the first lens 411 of the firstfeedback arm telescope until they strike the first reflectiondiffraction element 404. Upon striking the first reflection diffractionelement 404, the first feedback arm output 415 is split into separatecomponents.

A first component of the first feedback arm output 415 emerging from thefirst reflection diffraction element 404 is a reflection thatconstitutes a second feedback arm input 416. The second feedback arminput 416 propagates through a first lens 417 and a second lens 418 of asecond feedback arm telescope and towards a second feedback arm highlyreflective mirror 419. The second feedback arm input 416 is reflectedfrom the second feedback arm highly reflective mirror 419 as secondfeedback arm output 420. After reflecting from the second feedback armhighly reflective mirror 419, the constituent beams of the secondfeedback arm output 420 travel in a reverse direction through the secondlens 418 and the first lens 417 of the second feedback arm telescopeuntil they strike the first reflection diffraction element 404. Uponstriking the first reflection diffraction element 404, the secondfeedback arm output 420 is split into separate components. A firstcomponent is a reflection that serves as additional first feedback arminput 410. Therefore, a component of the beams continues traveling backand forth through the first feedback arm 435 and the second feedback arm440. Thus, an interferometer is formed by the first feedback arm highlyreflective mirror 414, the second feedback arm highly reflective mirror419, and the first reflection diffraction element 404. A secondcomponent is a reflective diffraction that serves as additional outputarm input 405.

A second component of the first feedback arm output 415 emerging fromthe first reflection diffraction element 404 is a first orderdiffraction that constitutes laser source resonant feedback 421. Thediffraction of the first feedback arm output 415 at the first reflectivediffraction grating 404 imparts an angular spectrum onto the lasersource resonant feedback 421. The laser source resonant feedback 421propagates through the position-to-angle transformation element 403towards the laser source 401 in a direction of propagation that isopposite that of the external resonator input 402. In the embodimentdepicted in FIG. 4, the feedback efficiency is the ratio of the opticalpower coupled into waveguides of laser source 401 to the total opticalpower carried by the laser source resonant feedback 421. In order topromote wavelength stabilization of the emitters of the laser source401, the feedback efficiency should be thirty percent or greater, andpreferably greater than seventy percent. The position-to-angletransformation element 403 transforms the angular spectrum of the lasersource resonant feedback into a position spectrum that corresponds tothe position of each emitter in the laser source 401. In that manner,each constituent beam of the laser source resonant feedback 421 isdirected into a single emitter of the laser source 401 therebystimulating emission of electromagnetic radiation corresponding to thepreferred resonant mode of each emitter of the laser source 401.

FIG. 5 illustrates an interferometric external resonator and beamcombining apparatus that utilizes a high diffraction efficiencyreflection diffraction element to provide resonant feedback for beamstabilization and to combine multiple single wavelength input beams intoa combined, multi-spectral output beam. FIG. 5 illustrates aninterferometric external resonator having three optical path arms: anemitter beam transformation arm 530, a first feedback arm 535, and asecond feedback arm 540. A laser source 501 is positioned at a firstterminus of the emitter beam transformation arm 530. The laser source501 includes a plurality of emitters located at fixed positions withrespect to one another. The position of each one of the plurality ofemitters defines a point in an emitter spatial position distribution.The laser source 501 emits a plurality of single wavelength, i.e. narrowspectral bandwidth, input beams that together constitute an externalresonator input 502. Upon exiting the laser source, the externalresonator input 502 has a position spectrum that corresponds to theemitter spatial position distribution. This enables each constituentbeam of the external resonator input 502 to be mapped to a particularemitter of the laser source 501 by its spatial position. The externalresonator input 502 propagates from the laser source towards areflection diffraction element 504 through a position-to-angletransformation element 503.

The position-to-angle transformation element 503 maps a position of eachsingle wavelength input beam to an angle of incidence with respect tothe reflection diffraction element 504. The position-to-angletransformation element 503 thereby transforms the position spectrum ofthe external resonator input 502 into an angular spectrum. Thus, uponemerging from the position-to-angle transformation element 503, theexternal resonator input 502 possesses an angular spectrum.Specifically, after emerging from the position-to-angle transformationelement 503, each constituent beam of the external resonator input 502and its corresponding emitter may be identified by an angle of incidencewith respect to the diffraction element 504. In the embodiment depictedin FIG. 5, the position-to-angle transformation element 503 is a Fourierlens. However, in alternative embodiments a variety of alternativetransformation optics, including Fresnel lenses, mirror arrangements,prismatic objects and diffractive gratings may be utilized as theposition-to-angle transformation element 503.

After propagating through the position-to-angle transformation element503, the external resonator input 502 is split into separate componentsupon striking the first reflection diffraction element 504. In theembodiment depicted in FIG. 5, the reflection diffraction element 504 isa high efficiency diffraction grating that acts as both an outputcombining element and a resonant feedback arm coupling element.Therefore, the majority of the power of an incident beam emerges fromthe interaction with the first reflection diffraction element as adiffraction, while a substantially smaller component emerges as areflection. In the embodiment depicted in FIG. 5, it is preferably thatsubstantially all of the power of an incident beam emerges from theinteraction with the diffraction grating as either a zero orderdiffraction or a first order diffraction. Furthermore, it is preferablethat the optical power carried by the first order diffraction is atleast four times the optical power carried by the zero orderdiffraction. In other words, in the embodiment depicted in FIG. 5, it isdesirable to select a diffraction grating that exhibits a first orderdiffraction efficiency of greater than 80% and simultaneously exhibits azero order diffraction efficiency of less than 20%.

A first component of the external resonator arm input 502 emerging fromthe reflection diffraction element 504 is a reflective diffraction ofthe resonator arm input 505 that constitutes a system output 505. Thesystem output 505 is composed of a plurality of spatially anddirectionally overlapped single wavelength beams emitted from theplurality of emitters of the laser source 501. The spatial locations ofthe laser source 501, the position-to-angle transformation element 503,and the reflection diffraction element 504 are fixed at precisepositions with respect to one another such that the components of theplurality of single wavelength beams emitted from the laser source 501that are emitted as system output 505 emerge from an overlap region ofthe reflection diffraction element 504 with a common direction ofpropagation.

A second component of the external resonator arm input 502 emerging fromthe reflection diffraction element 504 is a reflection that constitutesa first feedback arm input 506. The first feedback arm input 506consists of a plurality of single wavelength constituent beams. Thefirst feedback arm input 506 possesses an angular spectrum that is areflection of the angular spectrum imparted upon the external resonatorarm input 502 by the position-to-angle transformation element 503. Afteremerging from the reflection diffraction element 504, the first feedbackarm input 506 travels through a first feedback arm lens 507. The firstfeedback arm lens 507 directs the constituent beams of the firstfeedback arm input 506 at a first feedback arm high-reflective mirror508. Upon striking the first feedback arm high-reflective mirror 508,the constituent beams of the first feedback arm input 506 are reflectedas first feedback arm output 509. The first feedback arm output travelsback through the first feedback arm lens 507 and strikes the reflectivediffraction grating 504, which splits the first feedback arm output 509into separate components.

A first component of the first feedback arm output 509 emerging from thereflection diffraction element is a reflection that constitutesunfiltered laser source resonant feedback 518. A second component of thesecond feedback arm output 509 emerging from the reflection diffractionelement is a reflective diffraction that constitutes second feedback arminput 510. The second feedback arm input 510 includes a preferredresonant mode component 510A and an alternative resonant mode component510B. The preferred resonant mode component 510A is composed of aplurality of spatially and directionally overlapped single wavelengthconstituent beams that emerge from the reflection diffraction element504 with a common direction of propagation. Each constituent beam of thepreferred resonant mode component 510A is composed of electromagneticradiation corresponding to a preferred resonant mode of an emitter ofthe laser source 501. The alternative resonant mode component 510B iscomposed of a plurality of single wavelength constituent beams thatemerge from the reflection diffraction element 504 at an angle withrespect to the common direction of propagation of the preferred modecomponent 510A. Thus, the constituent beams of the alternative resonantmode component 510B are not spatially and directionally overlapped withrespect to each other or with respect to the constituent beams of thepreferred mode component 510A. Each constituent beam of the alternativeresonant mode component 510B is composed of electromagnetic radiationcorresponding to an alternative, non-preferred mode of an emitter of thelaser source 501. In other words, the alternative resonant modecomponent 510B consists of the unwanted spectral components of thesecond feedback arm input 510.

After emerging from the reflection diffraction grating 504, the secondfeedback arm input 510 travels through a first lens 511 of a secondfeedback arm telescope. The first lens 511 of the second feedback armtelescope directs the preferred mode component 510A through a spatialfiltering element 512 but directs the alternative resonant modecomponent 510B at from spatial filtering element 512 such that it isremoved from the optical path defining the external resonator. In thismanner, the alternative resonant mode component 510B is filtered out ofthe external resonator and prevented from being returned to the lasersource and thereby prevented from exciting alternative resonant modes ofthe emitters of the laser source 501. The electromagnetic radiationcomposing the alternative resonant mode component 510B may be absorbedby the material composing the spatial filtering element 512, or may bereflected from or transmitted through the spatial filtering element 512in a manner such that it is diverted from the optical path defining theexternal resonator.

After passing through the spatial filtering element, the preferred modecomponent 510A propagates through a second lens 513 of the secondfeedback arm telescope and towards a second feedback arm highlyreflective mirror 514. The preferred mode component 510A is reflectedfrom the second feedback arm highly reflective mirror 514 as secondfeedback arm output 515. Second feedback arm output 515 is a combinedbeam composed of a plurality of single wavelength constituent beams,wherein each constituent beam is composed of electromagnetic radiationcorresponding to a preferred resonant mode of an emitter of the lasersource 501. After reflecting from the second feedback arm highlyreflective mirror 514, the constituent beams of the second feedback armoutput 515 travel in a reverse direction through the second lens 513 ofthe second feedback arm telescope, through the spatial filtering element512, and through the first lens 511 of the second feedback arm telescopeuntil they strike the first reflection diffraction element 504.

Upon striking the first reflection diffraction element 504, the secondfeedback arm output 515 is split into separate components. A firstcomponent is a reflection that serves as additional system output 505. Asecond component is a diffraction that serves as filtered first feedbackarm input 516.

The filtered first feedback arm input 516 traverses the first feedbackarm and is reflected from the first feedback arm highly reflectivemirror 508 as filtered first feedback arm output 517. The filtered firstfeedback arm output 517 strikes the reflection diffraction element 504and is separated into components. A first component of the filteredfirst feedback arm output 517 emerging from the reflection diffractionelement 504 is a reflective diffraction that constitutes additionalsecond feedback arm input 510. Therefore, a component of the beamscontinues traveling back and forth through the first feedback arm 535and the second feedback arm 540. Thus, an interferometer is formed bythe first feedback arm highly reflective mirror 508, the second feedbackarm highly reflective mirror 514, and the first reflection diffractionelement 504. A second component of the filtered first feedback armoutput 517 emerging from the reflection diffraction element is areflection that constitutes filtered laser source resonant feedback 518.The laser source resonant feedback 518 propagates through theposition-to-angle transformation element 503 towards the laser source501 in a direction of propagation that is opposite that of the externalresonator input 502. The position-to-angle transformation element 503transforms the angular spectrum of the filtered laser source resonantfeedback 518 into a position spectrum that corresponds to the positionof each emitter in the laser source 501. In that manner, eachconstituent beam of the laser source resonant feedback 518 is directedinto a single emitter of the laser source 501 thereby stimulatingemission of electromagnetic radiation corresponding to the preferredresonant mode of each emitter of the laser source 501. In the embodimentdepicted in FIG. 5, the feedback efficiency is the ratio of the opticalpower coupled into waveguides of laser source 501 to the total opticalpower carried by the laser source resonant feedback 518. In order topromote wavelength stabilization of the emitters of the laser source501, the feedback efficiency should be thirty percent or greater, andpreferably greater than seventy percent.

FIG. 6 illustrates an additional interferometric external resonator andbeam combining apparatus that utilizes a high diffraction efficiencyreflection diffraction element with a backside thermal management systemto provide resonant feedback for beam stabilization and to combinemultiple single wavelength input beams into a combined, multi-spectraloutput beam. FIG. 6 illustrates an interferometric external resonatorand beam coupler having three optical path arms: an emitter beamtransformation arm 630, a first feedback arm 635, and a second feedbackarm 640. A laser source 601 is positioned at a first terminus of theemitter beam transformation arm 630. The laser source 601 includes aplurality of emitters located at fixed positions with respect to oneanother. The position of each one of the plurality of emitters defines apoint in an emitter spatial position distribution. The laser source 601emits a plurality of single wavelength, i.e. narrow spectral bandwidth,input beams that together constitute an external resonator input 602.

Upon exiting the laser source, the external resonator input 602 has aposition spectrum that corresponds to the emitter spatial positiondistribution. This enables each constituent beam of the externalresonator input 602 to be mapped to a particular emitter of the lasersource 601 by its spatial position. The external resonator input 602propagates from the laser source towards a reflection diffractionelement 604 through a position-to-angle transformation element 603. Theposition-to-angle transformation element 603 maps a position of eachsingle wavelength input beam to an angle of incidence with respect tothe reflection diffraction element 604. The position-to-angletransformation element 603 thereby transforms the position spectrum ofthe external resonator input 602 into an angular spectrum. Thus, uponemerging from the position-to-angle transformation element 603, theexternal resonator input 602 possesses an angular spectrum.Specifically, after emerging from the position-to-angle transformationelement 603, each constituent beam of the external resonator input 602and its corresponding emitter may be identified by an angle of incidencewith respect to the diffraction element 604. In the embodiment depictedin FIG. 6, the position-to-angle transformation element 603 is a Fourierlens. However, in alternative embodiments a variety of alternativetransformation optics, including Fresnel lenses, mirror arrangements,and diffractive gratings may be utilized as the position-to-angletransformation element 603.

After propagating through the position-to-angle transformation element603, the external resonator input 602 is split into separate componentsupon striking the first reflection diffraction element 604. In theembodiment depicted in FIG. 6, the reflection diffraction element 604 isa high-efficiency diffraction grating that acts as both an output beamcombining element and a resonant feedback arm coupling element.Therefore, the majority of the power of an incident beam emerges fromthe interaction with the first reflection diffraction element as adiffraction, while a substantially smaller component emerges as areflection. In the embodiment depicted in FIG. 6, it is preferably thatsubstantially all of the power of an incident beam emerges from theinteraction with the diffraction grating as either a zero orderdiffraction or a first order diffraction. Furthermore, it is preferablethat the optical power carried by the first order diffraction is atleast four times the optical power carried by the zero orderdiffraction. In other words, in the embodiment depicted in FIG. 6, it isdesirable to select a diffraction grating that exhibits a first orderdiffraction efficiency of greater than 80% and simultaneously exhibits azero order diffraction efficiency of less than 20%.

In the embodiment depicted in FIG. 6, the reflection diffraction element604 also includes a backside thermal management element 620. Thebackside thermal management element 620 removes heat generated byabsorption of electromagnetic radiation by the reflection diffractionelement 604. The backside thermal management element 620 facilitatessubstantially one dimensional heat-flow (in a direction perpendicular tothe plane in which the surface of the reflection diffraction element 604lies) and thereby reduces beam distortion attributable to the heating ofthe material from which the reflection diffraction element 604 isconstructed. One dimensional heat-flow further suppresses thermallensing effects and eliminates local differences in the grating constantthat result from thermal gradients induced by heating of the reflectiondiffraction element 604. One dimensional heat-flow can be achievedthrough selecting a diffractive element with an appropriatethickness-to-diameter ratio and the backside cooling element 620. Thebackside cooling element 620 is placed in “form-fitting” contact withthe reflection diffraction element 604 provides an additional benefit inthe form of providing mechanical stability for the reflectiondiffraction element 604 thereby preventing the element 604 from bendingunder thermal load. In the embodiment depicted in FIG. 6, the reflectiondiffraction element 604 is formed on a substrate with a high heatconductivity and a low coefficient of thermal expansion. For example thereflection diffraction element 604 may be formed from a substrate madeof diamond, sapphire, or a glass-ceramic such as Zerodur. The backsidethermal management element 620 may be made out of a material with highthermal conductivity, e.g. copper. The backside thermal managementelement 620 further has cooling channels 621. The cooling channels 621may be filled with a liquid with a high specific heat, such as water.The cooling channels 621 transport heat generated at the reflectiondiffraction element 604 and transmitted into the backside thermalmanagement element 620 away from the reflection diffraction element 604.One of skill in the art will appreciate that the cooling system depictedin FIG. 6 and described herein can be used with a variety of reflectiveoptical elements depicted in the preceding figures. For example, abackside cooling element (such as the backside cooling element 620)could be used with the first and second reflective diffraction elementsof FIGS. 1, 3, and 4 as well as with the reflective diffraction elementsof FIGS. 2 and 5.

A first component of the external resonator arm input 602 emerging fromthe reflection diffraction element 604 is a diffraction of the resonatorarm input 605 that constitutes a system output 605. The system output605 is composed of a plurality of spatially and directionally overlappedsingle wavelength beams emitted from the plurality of emitters of thelaser source 601. The spatial locations of the laser source 601, theposition-to-angle transformation element 603, and the reflectiondiffraction element 604 are fixed at precise positions with respect toone another such that the components of the plurality of singlewavelength beams emitted from the laser source 601 that are emitted assystem output 605 emerge from an overlap region of the reflectiondiffraction element 604 with a common direction of propagation.

A second component of the external resonator arm input 602 emerging fromthe reflection diffraction element 604 is a reflection that constitutesa first feedback arm input 606. The first feedback arm input 606consists of a plurality of single wavelength constituent beams. Thefirst feedback arm input 606 possesses an angular spectrum that is areflection of the angular spectrum imparted upon the external resonatorarm input 602 by the position-to-angle transformation element 603. Afteremerging from the reflection diffraction element 604, the first feedbackarm input 606 travels through a first feedback arm lens 607. The firstfeedback arm lens 607 directs the constituent beams of the firstfeedback arm input 606 at a first feedback arm high reflective mirror608. Upon striking the first feedback arm high-reflective mirror 608,the constituent beams of the first feedback arm input 606 are reflectedas first feedback arm output 609. The first feedback arm output travelsback through the first feedback arm lens 607 and strikes the reflectivediffraction element 604, which splits the first feedback arm output 609into separate components.

A first component of the first feedback arm output 609 emerging from thereflection diffraction element is a reflection that constitutesunfiltered laser source resonant feedback 618. A second component of thesecond feedback arm output 609 emerging from the reflection diffractionelement is a reflective diffraction that constitutes second feedback arminput 610. The second feedback arm input 610 includes a preferredresonant mode component 610A and an alternative resonant mode component610B. The preferred resonant mode component 610A is composed of aplurality of spatially and directionally overlapped single wavelengthconstituent beams that emerge from the reflection diffraction element604 with a common direction of propagation. Each constituent beam of thepreferred resonant mode component 610A is composed of electromagneticradiation corresponding to a preferred resonant mode of an emitter ofthe laser source 601. The alternative resonant mode component 610B iscomposed of a plurality of single wavelength constituent beams thatemerge from the reflection diffraction element 604 at an angle withrespect to the common direction of propagation of the preferred modecomponent 610A. Thus, the constituent beams of the alternative resonantmode component 610B are not spatially and directionally overlapped withrespect to each other or with respect to the constituent beams of thepreferred mode component 610A. Each constituent beam of the alternativeresonant mode component 610B is composed of electromagnetic radiationcorresponding to an alternative, non-preferred mode of an emitter of thelaser source 601.

After emerging from the reflection diffraction grating 604, the secondfeedback arm input 610 travels through a first lens 611 of a secondfeedback arm telescope. The first lens 611 of the second feedback armtelescope directs the preferred mode component 610A through a spatialfiltering element 612 but directs the alternative resonant modecomponent 610B at the spatial filtering element 612 such that itsremoved from the optical path defining the resonator. In this manner,the alternative resonant mode component 610B is filtered out of theexternal resonator and prevented from being returned to the laser 601source and thereby from exciting alternative resonant modes of theemitters. The electromagnetic radiation composing the alternativeresonant mode component 610B may be absorbed by the material composingthe spatial filtering element 612, or may be reflected from ortransmitted through the spatial filtering element 612 in a manner suchthat it is diverted from the optical path defining the externalresonator.

After passing through the spatial filtering element, the preferred modecomponent 610A propagates through a second lens 613 of the secondfeedback arm telescope and towards a second feedback arm highlyreflective mirror 614. The preferred mode component 610A is reflectedfrom the second feedback arm highly reflective mirror 614 as secondfeedback arm output 615. Second feedback arm output 615 is a combinedbeam composed of a plurality of single wavelength constituent beams,wherein each constituent beam is composed of electromagnetic radiationcorresponding to a preferred resonant mode of an emitter of the lasersource 601. After reflecting from the second feedback arm highlyreflective mirror 614, the constituent beams of the second feedback armoutput 615 travel in a reverse direction through the second lens 613 ofthe second feedback arm telescope, through the spatial filtering element612, and through the first lens 611 of the second feedback arm telescopeuntil they strike the first reflection diffraction element 604. Uponstriking the first reflection diffraction element 604, the secondfeedback arm output 615 is split into separate components. A firstcomponent is a reflection that serves as additional system output 605. Asecond component is a diffraction that serves as filtered first feedbackarm input 616.

The filtered first feedback arm input 616 traverses the first feedbackarm and is reflected from the first feedback arm highly reflectivemirror 608 as filtered first feedback arm output 617. The filtered firstfeedback arm output 617 strikes the reflection diffraction element 604and is separated into components. A first component of the filteredfirst feedback arm output 617 emerging from the reflection diffractionelement 604 is a reflective diffraction that constitutes additionalsecond feedback arm input 610. Therefore, a component of the beamscontinues traveling back and forth through the first feedback arm 635and the second feedback arm 640. Thus, an interferometer is formed bythe first feedback arm highly reflective mirror 608, the second feedbackarm highly reflective mirror 614, and the first reflection diffractionelement 604. A second component of the filtered first feedback armoutput 617 emerging from the reflection diffraction element is areflection that constitutes filtered laser source resonant feedback 618.The laser source resonant feedback 618 propagates through theposition-to-angle transformation element 603 towards the laser source601 in a direction of propagation that is opposite that of the externalresonator input 602. The position-to-angle transformation element 603transforms the angular spectrum of the filtered laser source resonantfeedback 618 into a position spectrum that corresponds to the positionof each emitter in the laser source 601. In that manner, eachconstituent beam of the laser source resonant feedback 618 is directedinto a single emitter of the laser source 601 thereby stimulatingemission of electromagnetic radiation corresponding to the preferredresonant mode of each emitter of the laser source 601. In the embodimentdepicted in FIG. 6, the feedback efficiency is the ratio of the opticalpower coupled into waveguides of laser source 601 to the total opticalpower carried by the laser source resonant feedback 618. In order topromote wavelength stabilization of the emitters of the laser source601, the feedback efficiency should be thirty percent or greater, andpreferably greater than seventy percent.

It is thus contemplated that other implementations of the invention maydiffer in detail from foregoing examples. As such, all references to theinvention are intended to reference the particular example of theinvention being discussed at that point in the description and are notintended to imply any limitation as to the scope of the invention moregenerally. All language of distinction and disparagement with respect tocertain features is intended to indicate a lack of preference for thosefeatures, but not to exclude such from the scope of the inventionentirely unless otherwise indicated.

The use of the terms “a” and “an” and “the” and similar referents in thecontext of describing the invention (especially in the context of thefollowing claims) are to be construed to cover both the singular and theplural, unless otherwise indicated herein or clearly contradicted bycontext. The terms “comprising,” “having,” “including,” and “containing”are to be construed as open-ended terms (i.e., meaning “including, butnot limited to”) unless otherwise noted. Recitation of ranges of valuesherein are merely intended to serve as a shorthand method of referringindividually to each separate value falling within the range, unlessotherwise indicated herein, and each separate value is incorporated intothe specification as if it were individually recited herein. All methodsdescribed herein can be performed in any suitable order unless otherwiseindicated herein or otherwise clearly contradicted by context. The useof any and all examples, or exemplary language (e.g., “such as”)provided herein, is intended merely to better illuminate the inventionand does not pose a limitation on the scope of the invention unlessotherwise claimed. No language in the specification should be construedas indicating any non-claimed element as essential to the practice ofthe invention.

Accordingly, this invention includes all modifications and equivalentsof the subject matter recited in the claims appended hereto as permittedby applicable law. Moreover, any combination of the above-describedelements in all possible variations thereof is encompassed by theinvention unless otherwise indicated herein or otherwise clearlycontradicted by context.

1. A system for stabilizing the wavelength of beams emitted by aplurality of beam emitters, the system comprising: the plurality of beamemitters, each beam emitter emitting a beam; a first reflectiondiffraction element; and a feedback branch comprising a spatialfiltering system; wherein the first reflection diffraction elementdirects a portion of the beams originating at the array into thefeedback branch as feedback branch input, and wherein the feedbackbranch directs a portion of the feedback branch input back into theplurality of beam emitters.
 2. The system of claim 1, wherein the firstreflection diffraction element receives the beams originating from theplurality of beam emitters and directs, into the feedback branch as afeedback branch input, a diffraction of the beams originating from thearray.
 3. The system of claim 2, the system further comprising: a secondreflection diffraction element; wherein the second reflectiondiffraction element receives, from the first reflection diffractionelement, a reflection of the beams originating from the plurality ofbeam emitters, and wherein the second diffraction element directs, as asystem output, a diffraction of the reflection of the beams originatingfrom the plurality of beam emitters.
 4. The beam stabilization system ofclaim 1, wherein the first reflection diffraction element receives thebeams originating from the plurality of beam emitters and directs, intothe feedback branch as a feedback branch input, a reflection of thebeams originating from the plurality of beam emitters.
 5. The system ofclaim 4, wherein the first reflection diffraction element receives thebeams originating from the plurality of beam emitters and directs, as asystem output, a diffraction of the beams originating from the pluralityof beam emitters.
 6. The system of claim 1, the system furthercomprising: a cooling element disposed on the first reflectiondiffraction element.
 7. The system of claim 6, wherein the coolingelement is disposed on a side of the first reflection diffractionelement opposite a side of the first reflection diffraction element thatdirects a portion of the beams originating at the array into thefeedback branch as feedback branch input.
 8. The system of claim 6,wherein the cooling element contains one or more cooling channels,wherein the cooling channels are configured to transport heat away fromthe first reflection diffraction element.
 9. The system of claim 8,wherein the cooling channel is filled with water.
 10. The system ofclaim 3, the system further comprising: a cooling element disposed onthe second reflection diffraction element.
 11. The system of claim 10,wherein the cooling element is disposed on a side of the firstreflection diffraction element opposite a side of the second reflectiondiffraction element that directs a portion of the beams originating atthe array into the feedback branch as a feedback branch input.
 12. Thesystem of claim 10, wherein the cooling element contains one or morecooling channels, wherein the cooling channels are configured totransport heat away from the second reflection diffraction element. 13.The system of claim 12, wherein the cooling channel is filled withwater.
 14. The system of claim 1, wherein the first order diffractionefficiency of the first reflection diffraction element is less than 20%.15. The system of claim 1, wherein the first order diffractionefficiency of the first reflection diffraction element is greater than80%.
 16. The system of claim 1, wherein the plurality of beam emittersis an array of diode beam emitters.
 17. The system of claim 16, whereineach beam emitter in the array of diode beam emitters emits a beam withan asymmetric profile, wherein each beam has a fast axis and a slowaxis.
 18. The system of claim 1, wherein each emitter in the pluralityof beam emitters comprises a waveguide, and wherein at least thirtypercent of the optical power of the portion of the feedback branch inputdirected back into the plurality of beam emitters is coupled into theplurality of waveguides.
 19. The system of claim 1, wherein each emitterin the plurality of beam emitters comprises a waveguide, and wherein atleast seventy percent of the optical power of the portion of thefeedback branch input directed back into the plurality of beam emittersis coupled into the plurality of waveguides.
 20. The system of claim 1,further comprising: a plurality of optical elements forming an emitterbeam processing branch; wherein the emitter beam processing branch islocated between the plurality of beam emitters and the first reflectiondiffraction element, and wherein the emitter beam processing branchreceives the beams emitted by the plurality of beam emitters and directsthe beams emitted by the plurality of beam emitters towards the firstreflection diffraction element.
 21. The system of claim 20, theplurality of optical elements forming the emitter beam processing branchcomprising: a cylindrical fast-axis collimation optic; and a cylindricalslow-axis collimation optic wherein the cylindrical fast-axiscollimation optic collimates the beams emitted by the array of beamemitters along the fast-axis of the beams, and wherein the cylindricalslow-axis collimation optic collimates the beams emitted by the array ofbeam emitters along the slow-axis of the beams.
 22. The system of claim21, the plurality of optical elements forming the emitter beamprocessing branch further comprising: a beam rotator, wherein the beamrotator rotates each beam emitted by the array of beam emitters byninety degrees.
 23. The system of claim 21, the plurality of opticalelements forming the emitter beam processing branch further comprising:a cylindrical optic, wherein the cylindrical optic focuses each beamemitted by the plurality of beam emitters to a specific point on thefirst reflection diffraction element.
 24. The system of claim 1, whereinthe spatial filtering system comprises one of the group consisting of:an aperture, a diaphragm, a waveguide structure, and a set of mirrorsthat include a gradient layer.
 25. The system of claim 24, wherein thespatial filtering system further comprises a telescope, the telescopecomprising a first optic and a second optic.
 26. A method forstabilizing the wavelength of beams emitted by a plurality of beamemitters, the method comprising: directing the emitted beams towards afirst reflection diffraction element; directing a portion of the emittedbeams from the first reflection diffraction element into a feedbackbranch as a feedback branch input; and directing a portion of thefeedback branch input through the feedback branch and back into theplurality of beam emitters; wherein directing a portion of the feedbackbranch input through the feedback branch comprises sequentiallydirecting a portion of the feedback branch input through a spatialfiltering system.
 27. The method of claim 26, wherein the directing,from the first reflection diffraction element into a feedback branch asfeedback branch input, a portion of the emitted beams comprises:directing, from the first reflection diffraction element, a diffractionof the emitted beams into the feedback branch as the feedback branchinput.
 28. The method of claim 27, the method further comprising:directing, from the first reflection diffraction element, a reflectionof the emitted beams towards a second reflection diffraction element asan output branch input; and directing, from the second reflectiondiffraction element, a diffraction of the output branch input as anoutput beam.
 29. The method of claim 28, wherein the output beamcomprises a plurality of spatially overlapped single wavelength beams.30. The method of claim 29, wherein each one of the plurality ofspatially overlapped single wavelength beams corresponds to a singlebeam emitter in the plurality of beam emitters.
 31. The method of claim26, wherein the directing, from the first reflection diffraction elementinto a feedback branch as feedback branch input, a portion of theemitted beams comprises: directing, from the first reflectiondiffraction element, a reflection of the emitted beams into the feedbackbranch as the feedback branch input.
 32. The method of claim 31, themethod further comprising: directing, from the first reflectiondiffraction element, a diffraction of the emitted beams as an outputbeam.
 33. The method of claim 32, wherein the output beam comprises aplurality of spatially overlapped single wavelength beams.
 34. Themethod of claim 33, wherein each one of the plurality of spatiallyoverlapped single wavelength beams corresponds to a single beam emitterin the array of beam emitters.
 35. The method of claim 26, wherein thedirecting a portion of the feedback branch input through the feedbackbranch and back into the array of beam emitters comprises: directing thefeedback branch input into a first arm of the feedback branch as a firstarm input; directing the first arm input towards a first high reflectivemirror; directing a reflection of the first arm input from the firsthigh reflective mirror towards the first reflection diffraction gratingas a first arm output; directing a reflection of the first arm outputtowards a second high reflective mirror of a second feedback arm as asecond arm input; directing a reflection of the second arm input fromthe second high reflective mirror towards the first reflectiondiffraction element as a second arm output; directing a reflection ofthe second arm output into the first arm of the feedback branch as afirst arm input; and directing a diffraction of the first arm outputback into the array of beam emitters.
 36. The method of claim 26,wherein the directing a portion of the feedback branch input through thefeedback branch and back into the array of beam emitters comprises:directing the feedback branch input into a first arm of the feedbackbranch as a first arm input; directing the first arm input towards afirst high reflective mirror; directing a reflection of the first arminput from the first high reflective mirror towards the first reflectiondiffraction element as a first arm output; directing a diffraction ofthe first arm output from the first reflection diffraction element intoa second arm of the feedback branch as a second arm input; directing thesecond arm input towards a second high reflective mirror; directing thereflection of the second arm input from the second high reflectivemirror towards the first reflection diffraction element as a second armoutput; directing a diffraction of the second arm output into the firstarm towards the first high reflective mirror as a first arm input;directing a reflection of the first arm input from the first reflectiondiffraction element back into the plurality of beam emitters.
 37. Adense wavelength multiplexing system comprising: an array of beamemitters each emitting a single wavelength beam; a first diffractionelement reflecting the plurality of single wavelength beams from thearray of beam emitters; and a second diffraction element diffracting thereflection of the plurality of single wavelength beams so as to combinethe beams into a single multi-wavelength combined beam.
 38. A densewavelength multiplexing system comprising: a plurality of beam emitterseach emitting a single wavelength beam; a first diffraction elementdiffracting a portion of the beams from the plurality of beam emittersso as to combine the beams into a single multi-wavelength combinedoutput beam.
 39. A dense wavelength multiplexing and beam wavelengthstabilization system comprising: a plurality of beam emitters eachemitting a beam; at least one optical element; a first reflectiondiffraction grating; a second reflection diffraction grating; and afeedback branch having a first arm and a second arm, wherein the firstreflection diffraction grating directs a reflection of the beams towardsthe second diffraction grating and directs a diffraction of the beamsinto the first arm of the feedback branch as a first arm input, whereinthe second diffraction grating receives the reflection of the beams anddiffracts the reflection as an output, wherein the first arm of thefeedback branch includes a first highly reflective mirror positioned toreflect the first arm input back toward the first reflection diffractiongrating as a first arm output such that the first reflection diffractiongrating diffracts a first portion of the first arm output back to thearray of beam emitters as a first feedback portion and reflects a secondportion of the first arm output into the second arm as a second arminput, and wherein the second arm includes a second highly reflectivemirror positioned to reflect the second arm input back to the firstreflection diffraction grating as a second arm output such that thefirst reflection diffraction grating reflects a first portion of thesecond arm output into the first arm and reflectively diffracts a secondportion of the second output arm output towards the second diffractiongrating.
 40. A dense wavelength multiplexing and beam wavelengthstabilization system comprising: a plurality of beam emitters eachemitting a beam; at least one optical element; a reflection diffractiongrating; and a feedback branch having a first arm and a second arm,wherein the reflection diffraction grating directs a reflection of thebeams into the first arm of the feedback branch as a first arm input anddirects a diffraction of the beams as a system output, wherein the firstarm of the feedback branch includes a first highly reflective mirrorpositioned to reflect the first arm input back toward the reflectiondiffraction grating as a first arm output such that the reflectiondiffraction grating reflects a first portion of the first arm outputback to the array of beam emitters as a first feedback portion anddiffracts a second portion of the first arm output into the second armas a second arm input, and wherein the second arm includes a secondhighly reflective mirror positioned to reflect the second arm input backto the reflection diffraction grating as a second arm output such thatthe reflection diffraction grating reflects a first portion of thesecond arm output as an additional system output and diffracts a secondportion of the second output arm output into the first arm of thefeedback branch as an additional first arm input.